Oncoproteins, the products encoded by oncogenes, can induce cancer in animals and transform normal cells in culture to become cancerous cells. Many oncoproteins are either derivatives of, or identical to, normal cellular proteins that regulate cell growth and division. Oncoproteins arise by two means. First, a mutation in the DNA sequence encoding a normal cellular protein can result in a mutant protein and a subsequent loss of cellular growth control. Second, a wildtype protein can be expressed at abnormally high levels or at inappropriate times due to a change in transcriptional regulation.
Oncoproteins induce changes in the growth characteristics of cells, resulting in tumor formation if these cells are injected into animals. An alteration that leads to increased tumor-forming capacity is termed a “malignant transformation.” Often, transformation requires the synergistic effect of multiple oncogenes acting at once rather than a single mutation. The result of malignant transformation is generally cell immortality. When normal cells become damaged, hazardous, or superfluous, they undergo a programmed cell death known as apoptosis. A cell's mortality is believed to be an outcome of the differentiation of a dividing stem cell into an end-stage, non-dividing cell. Immortal cancerous cells may block this differentiation and remain in a continually dividing state, resulting in unlimited growth potential. Other characteristics of transformed cells include alterations in growth parameters and cell behavior, alterations at the cell surface, loss of cytoskeletal elements, secretion of transforming growth factors and proteases, and altered gene transcription. Transformed cells continue to divide when a normal cell would not, resulting in an increased saturation density in culture. Other changes include decreased growth factor and hormone requirements, deficiency in capacity for growth arrest, anchorage independent growth, a loss of contact inhibition, and altered cellular morphology.
Growth factors, growth factor receptors, intracellular signal transducers, nuclear transcription factors, and cell-cycle control proteins are some common oncoprotein candidates. Growth factors exist extracellularly and function as ligands for their associated receptors. The binding of a growth factor to its receptor normally induces a response that will initiate cell division. The receptor can be either intracellular, requiring a steroid or hormone growth factor, or bound to the cell membrane. When a membrane bound receptor contacts its ligand, the receptor stimulates a cascade response known as signal transduction. Signals are usually transmitted by the phosphorylation of numerous proteins by an activated kinase. The end result of this process is often regulation of gene transcription through the activation or inactivation of nuclear transcription factors by a change in the phosphorylation state. Transcription factors exert control over gene regulation by binding to DNA at specific sequence motifs within the promoters or enhancers of target genes.
Oct-1 is a ubiquitously expressed transcription factor that binds to an eight nucleotide sequence (ATTTGCAT) in the promoters and enhancers of a number of genes. It is a member of a family of transcription factors containing a POU domain, a 155 to 162 amino acid region which consists of a bipartite DNA-binding domain. This domain contains both an N-terminal POUS sub-domain and a C-terminal POUHD homeodomain. Oct-1 has been shown to regulate the transcription of many genes, including those encoding small nuclear RNAs, immunoglobulin, and the histone, H2B. Histones are essential for DNA synthesis and play an important role in controlling cell cycle progression. During the S phase of the cell cycle, transcription of the H2B histone gene requires the interaction of Oct-1 with its binding motif in the H2B promoter. In developing frog embryos undergoing rapid division, it has been shown that Oct-1 must be bound to the octamer motif, along with other factors bound to their respective sites, for maximal H2B transcription. This effect is not due to an increase in the DNA binding affinity of Oct-1, but rather to changes in transcription factor interactions at the octamer motif and at the CCAAT regulatory element in the promoter. In addition, it has been shown that Oct-1 is phosphorylated at a site in the homeodomain upon entering mitosis. This phosphorylation reduces the ability of Oct-1 to bind DNA. As cells exit mitosis, Oct-1 is subsequently dephosphorylated, thereby restoring its DNA binding affinity. These observations are consistent with the transcriptional inhibition at several Oct-1 regulated genes during mitosis, again illustrating the protein's importance in cell cycle control.
Oct-1 also plays a role in DNA synthesis in adenoviral replication. Upon infection with an adenovirus, the binding activity of Oct-1 is stimulated. This most likely occurs due to a decrease in phosphorylation of Oct-1 resulting in an increase in the level of the active protein. Octamer motifs are found in the viral terminal repeat sequences, which are important for the initiation of DNA replication. Indeed, it was shown previously that Oct-1 increases DNA replication in the adenovirus up to six-fold due to increased initiation frequency.